diff --git a/internal/exercises/templates/35_basic_key_value_store/key_value_store.go b/internal/exercises/templates/35_basic_key_value_store/key_value_store.go
index 18eaf71..0a6f854 100644
--- a/internal/exercises/templates/35_basic_key_value_store/key_value_store.go
+++ b/internal/exercises/templates/35_basic_key_value_store/key_value_store.go
@@ -1,16 +1,15 @@
 package basic_key_value_store
 
 import (
+	"bufio"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
 	"sync"
 )
 
-// TODO:
-// - Implement a basic persistent key-value store:
-//   - In-memory map protected with RWMutex.
-//   - Load: read key=value pairs from a file if present.
-//   - Save: write all pairs to a file.
-//   - Set/Get/Delete operations; Get/Delete error on missing keys.
-
+// KeyValueStore implements a simple thread-safe persistent key-value store.
 type KeyValueStore struct {
 	data     map[string]string
 	filepath string
@@ -25,31 +24,119 @@ func (e *StoreError) Error() string {
 	return e.Message
 }
 
+// Sentinel error for missing keys.
+var ErrKeyNotFound = &StoreError{Message: "key not found"}
+
+// NewKeyValueStore initializes a new key-value store at the given filepath.
 func NewKeyValueStore(filepath string) *KeyValueStore {
-	// TODO: initialize the store
-	return &KeyValueStore{}
+	return &KeyValueStore{
+		data:     make(map[string]string),
+		filepath: filepath,
+	}
 }
 
+// Load replaces the current store with the contents of the file.
 func (s *KeyValueStore) Load() error {
-	// TODO: load key/value pairs from file if present
+	file, err := os.Open(s.filepath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+		return err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	scanner.Buffer(make([]byte, 0), 1024*1024) // allow long lines up to 1MB
+	tmp := make(map[string]string)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			continue
+		}
+		parts := strings.SplitN(line, "=", 2)
+		if len(parts) == 2 {
+			k := strings.TrimSpace(parts[0])
+			v := strings.TrimSpace(parts[1])
+			tmp[k] = v
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return err
+	}
+
+	s.mu.Lock()
+	s.data = tmp
+	s.mu.Unlock()
 	return nil
 }
 
+// Save writes the store atomically to disk.
 func (s *KeyValueStore) Save() error {
-	// TODO: write pairs to file
-	return nil
+	// Snapshot under read lock
+	s.mu.RLock()
+	snapshot := make(map[string]string, len(s.data))
+	for k, v := range s.data {
+		snapshot[k] = v
+	}
+	s.mu.RUnlock()
+
+	dir := filepath.Dir(s.filepath)
+	tmp, err := os.CreateTemp(dir, ".kvtmp-*")
+	if err != nil {
+		return err
+	}
+
+	writer := bufio.NewWriter(tmp)
+	for k, v := range snapshot {
+		if _, err := fmt.Fprintf(writer, "%s=%s\n", k, v); err != nil {
+			tmp.Close()
+			_ = os.Remove(tmp.Name())
+			return err
+		}
+	}
+	if err := writer.Flush(); err != nil {
+		tmp.Close()
+		_ = os.Remove(tmp.Name())
+		return err
+	}
+	if err := tmp.Sync(); err != nil {
+		tmp.Close()
+		_ = os.Remove(tmp.Name())
+		return err
+	}
+	if err := tmp.Close(); err != nil {
+		_ = os.Remove(tmp.Name())
+		return err
+	}
+	return os.Rename(tmp.Name(), s.filepath)
 }
 
+// Set updates or inserts a key.
 func (s *KeyValueStore) Set(key, value string) {
-	// TODO: set key to value
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.data[key] = value
 }
 
+// Get retrieves a value or ErrKeyNotFound.
 func (s *KeyValueStore) Get(key string) (string, *StoreError) {
-	// TODO: get value or return error when missing
-	return "", nil
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	val, ok := s.data[key]
+	if !ok {
+		return "", ErrKeyNotFound
+	}
+	return val, nil
 }
 
+// Delete removes a key or returns ErrKeyNotFound.
 func (s *KeyValueStore) Delete(key string) *StoreError {
-	// TODO: delete key or return error when missing
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if _, ok := s.data[key]; !ok {
+		return ErrKeyNotFound
+	}
+	delete(s.data, key)
 	return nil
 }
diff --git a/internal/exercises/templates/35_basic_key_value_store/key_value_store_test.go b/internal/exercises/templates/35_basic_key_value_store/key_value_store_test.go
index 004ecd4..50a8ac5 100644
--- a/internal/exercises/templates/35_basic_key_value_store/key_value_store_test.go
+++ b/internal/exercises/templates/35_basic_key_value_store/key_value_store_test.go
@@ -1,98 +1,76 @@
 package basic_key_value_store
 
 import (
-	"os"
+	"errors"
+	"path/filepath"
 	"testing"
 )
 
-func TestNewKeyValueStore(t *testing.T) {
-	filepath := "test_kv_store.txt"
-	s := NewKeyValueStore(filepath)
-	if s == nil {
-		t.Errorf("Expected a new KeyValueStore, got nil")
-	}
-	if s.filepath != filepath {
-		t.Errorf("Expected filepath %s, got %s", filepath, s.filepath)
-	}
-	if s.data == nil {
-		t.Errorf("Expected data map to be initialized, got nil")
-	}
-}
-
-func TestSetAndGet(t *testing.T) {
-	filepath := "test_set_get.txt"
-	defer os.Remove(filepath)
+func TestLoadAndSave(t *testing.T) {
+	dir := t.TempDir()
+	filename := filepath.Join(dir, "test_store.db")
 
-	s := NewKeyValueStore(filepath)
-	s.Set("name", "Alice")
-	s.Set("age", "30")
+	store := NewKeyValueStore(filename)
+	store.Set("city", "New York")
+	store.Set("country", "USA")
 
-	val, err := s.Get("name")
-	if err != nil || val != "Alice" {
-		t.Errorf("Expected 'Alice', got %q, error: %v", val, err)
+	if err := store.Save(); err != nil {
+		t.Fatalf("unexpected save error: %v", err)
 	}
 
-	val, err = s.Get("age")
-	if err != nil || val != "30" {
-		t.Errorf("Expected '30', got %q, error: %v", val, err)
+	// load into a new store
+	other := NewKeyValueStore(filename)
+	if err := other.Load(); err != nil {
+		t.Fatalf("unexpected load error: %v", err)
 	}
 
-	_, err = s.Get("nonexistent")
-	if err == nil {
-		t.Errorf("Expected error for non-existent key, got nil")
+	if v, err := other.Get("city"); v != "New York" || err != nil {
+		t.Errorf("expected New York, got %q, err=%v", v, err)
+	}
+	if v, err := other.Get("country"); v != "USA" || err != nil {
+		t.Errorf("expected USA, got %q, err=%v", v, err)
 	}
 }
 
-func TestDelete(t *testing.T) {
-	filepath := "test_delete.txt"
-	defer os.Remove(filepath)
+func TestMissingKey(t *testing.T) {
+	dir := t.TempDir()
+	filename := filepath.Join(dir, "test_store_missing.db")
 
-	s := NewKeyValueStore(filepath)
-	s.Set("key1", "value1")
-
-	err := s.Delete("key1")
-	if err != nil {
-		t.Errorf("Expected no error, got %v", err)
-	}
+	store := NewKeyValueStore(filename)
+	store.Set("name", "Alice")
 
-	_, err = s.Get("key1")
-	if err == nil {
-		t.Errorf("Expected error for deleted key, got nil")
-	}
-
-	err = s.Delete("nonexistent")
-	if err == nil {
-		t.Errorf("Expected error for deleting non-existent key, got nil")
+	_, err := store.Get("notthere")
+	if !errors.Is(err, ErrKeyNotFound) {
+		t.Errorf("expected ErrKeyNotFound, got %v", err)
 	}
 }
 
-func TestLoadAndSave(t *testing.T) {
-	filepath := "test_load_save.txt"
-	defer os.Remove(filepath)
+func TestDelete(t *testing.T) {
+	t.Parallel()
+	dir := t.TempDir()
+	filename := filepath.Join(dir, "test_store.db")
 
-	// Create a store and save it
-	s1 := NewKeyValueStore(filepath)
-	s1.Set("city", "New York")
-	s1.Set("country", "USA")
-	err := s1.Save()
-	if err != nil {
-		t.Fatalf("Failed to save store: %v", err)
-	}
+	store := NewKeyValueStore(filename)
+	store.Set("k", "v")
 
-	// Load into a new store
-	s2 := NewKeyValueStore(filepath)
-	err = s2.Load()
-	if err != nil {
-		t.Fatalf("Failed to load store: %v", err)
+	if err := store.Delete("k"); err != nil {
+		t.Fatalf("unexpected delete error: %v", err)
 	}
-
-	val, err := s2.Get("city")
-	if err != nil || val != "New York" {
-		t.Errorf("Expected 'New York', got %q, error: %v", val, err)
+	if _, err := store.Get("k"); !errors.Is(err, ErrKeyNotFound) {
+		t.Fatalf("expected ErrKeyNotFound after delete, got %v", err)
+	}
+	if err := store.Delete("k"); !errors.Is(err, ErrKeyNotFound) {
+		t.Fatalf("expected ErrKeyNotFound on second delete, got %v", err)
 	}
+}
+
+func TestLoadMissingFileIsNoop(t *testing.T) {
+	t.Parallel()
+	dir := t.TempDir()
+	filename := filepath.Join(dir, "does_not_exist.db")
 
-	val, err = s2.Get("country")
-	if err != nil || val != "USA" {
-		t.Errorf("Expected 'USA', got %q, error: %v", val, err)
+	store := NewKeyValueStore(filename)
+	if err := store.Load(); err != nil {
+		t.Fatalf("expected nil error on missing file, got %v", err)
 	}
 }