Lucy Baines feels neglected by her dysfunctional family. Her father Walt is lazy, her mother Joy is preoccupied, and her brother Gary is self-absorbed. Feeling lonely, Lucy revives her childhood imaginary friend Zara, who manifests as an idealized version of a caring family.
Zara convinces Lucy to make her real family disappear, promising a perfect replacement family. When Lucy follows Zara's instructions, her actual family vanishes, replaced by Zara's imaginary family. However, this new family turns out to be a nightmare, treating Lucy poorly.
Realizing her mistake, Lucy wishes for her real family back, leading to a confrontation and her getting locked out. She is forced to break back into her own home, resulting in a fight where she falls down the stairs and hits her head.
Upon waking up, Lucy finds her real family worried about her, and they start communicating better for a while.

Ayckbourn employs a clever theatrical device where the audience can see Zara, but Lucy's family cannot, inverting the usual convention of an imaginary friend only visible to a child.

The play highlights how Lucy uses her invisible friend as a coping mechanism to deal with her parents' crumbling marriage and her family's inability to communicate openly.

"Invisible Friends" exemplifies Ayckbourn's talent for dark comedies that explore the adversities and dysfunction lurking beneath the veneer of suburban family life.

The play is considered one of Ayckbourn's "offstage action" works, where the real drama happens offstage or through absent characters, reflecting his characters' inability to confront their problems head-on.

In summary, "Invisible Friends" inventively uses the concept of an imaginary companion to shed light on a troubled teenager's struggles within a dysfunctional household, showcasing Ayckbourn's mastery of farcical yet poignant family dramas.