diff --git a/composite.go b/composite.go
new file mode 100644
index 0000000..8ea9c4b
--- /dev/null
+++ b/composite.go
@@ -0,0 +1,68 @@
+package monstersirenfetch
+
+import (
+	"os"
+	"strconv"
+)
+
+// AlbumConflictError is returned by [Flatten] if two or more albums share the same CID.
+type AlbumConflictError struct {
+	Index    int
+	Previous Album
+	Current  Album
+}
+
+func (e *AlbumConflictError) Error() string {
+	return "album CID conflict on index " + strconv.Itoa(e.Index) + ": " +
+		"previous album " + e.Previous.Name + ", " +
+		"current album " + e.Current.Name
+}
+
+// SongConflictError is returned by [Flatten] if two or more songs belonging to the same [Album] share the same CID.
+type SongConflictError struct {
+	Index    int
+	Previous *Song
+	Current  Song
+}
+
+func (e *SongConflictError) Error() string {
+	return "song CID conflict on index " + strconv.Itoa(e.Index) + ": " +
+		"previous song " + e.Previous.Name + ", " +
+		"current song " + e.Current.Name
+}
+
+// CompositeAlbum represents an [Album] with a collection of its associated identifier-only [Song].
+type CompositeAlbum struct {
+	// Songs is a map of [Song.CID] to identifier-only [Song].
+	Songs map[StringInt]*Song
+
+	*Album
+}
+
+// CompositeAlbumsMap is a map of [Album.CID] to [CompositeAlbum].
+type CompositeAlbumsMap map[StringInt]CompositeAlbum
+
+// Flatten flattens [AlbumsData] and [SongsData] into a [CompositeAlbumsMap].
+// All values in [CompositeAlbum.Songs] and the [CompositeAlbum.Album] field are guaranteed to be non-nil.
+func Flatten(albumData AlbumsData, songsData SongsData) (CompositeAlbumsMap, error) {
+	m := make(CompositeAlbumsMap, len(albumData))
+	for i, a := range albumData {
+		if c, ok := m[a.CID]; ok {
+			return nil, &AlbumConflictError{Index: i, Previous: *c.Album, Current: a}
+		}
+		m[a.CID] = CompositeAlbum{Songs: make(map[StringInt]*Song), Album: &a}
+	}
+
+	for i, s := range songsData.List {
+		var c *Song
+		if a, ok := m[s.AlbumCID]; !ok {
+			return nil, os.ErrNotExist
+		} else if c, ok = a.Songs[s.CID]; ok {
+			return nil, &SongConflictError{Index: i, Previous: c, Current: s}
+		} else {
+			a.Songs[s.CID] = &s
+		}
+	}
+
+	return m, nil
+}
diff --git a/composite_test.go b/composite_test.go
new file mode 100644
index 0000000..afd1738
--- /dev/null
+++ b/composite_test.go
@@ -0,0 +1,31 @@
+package monstersirenfetch_test
+
+import (
+	"encoding/json"
+	"testing"
+
+	. "git.gensokyo.uk/yonah/monstersirenfetch"
+)
+
+func TestFlatten(t *testing.T) {
+	t.Run("sample", func(t *testing.T) {
+		var (
+			albumsResp AlbumsResponse
+			songsResp  SongsResponse
+		)
+
+		if err := json.Unmarshal(albumsJSON, &albumsResp); err != nil {
+			t.Fatalf("Unmarshal: error = %v", err)
+		}
+		if err := json.Unmarshal(songsJSON, &songsResp); err != nil {
+			t.Fatalf("Unmarshal: error = %v", err)
+		}
+
+		if m, err := Flatten(albumsResp.Data, songsResp.Data); err != nil {
+			t.Fatalf("Flatten: error = %v", err)
+		} else {
+			// TODO(ophestra): validate this result
+			t.Logf("Flatten: %#v", m)
+		}
+	})
+}
diff --git a/song.go b/song.go
index 5190fc5..31faae3 100644
--- a/song.go
+++ b/song.go
@@ -9,6 +9,7 @@ import (
 type SongResponse Response[Song]
 
 // Song holds the metadata of a song.
+// Fields marked with omitempty are only populated when the IsFull method returns true.
 type Song struct {
 	CID        StringInt `json:"cid"`
 	Name       string    `json:"name"`