(AP) -- Ally Sheedy as a mental patient. Former Brat Packer Ally Sheedy is featured in tonight's episode of "Once and Again." It's on ABC. She plays a hot-headed mental patient who's seeing Lily's sensitive, but troubled, brother, Aaron

Lily's brother expresses a desire to move in with a young woman who, like him, is mentally impaired. (PG)

Movie veteran Ally Sheedy hasn't made many appearances on TV, so if you're a fan of the star of The Breakfast Club and St. Elmo's Fire, you'd do well to tune in to Once and Again. Sheedy's guest role sees her playing a mentally disturbed woman looking to move in with Aaron (Patrick Dempsey). But the fact that Aaron has emotional problems of his own causes Lily (Sela Ward) to become concerned about the couple's future.

The "Aaron's List of Dreams" episode. When Lily's (Sela Ward) brother Aaron (Dempsey) expresses a desire to take part in an independent living program with a young woman who is also mentally disabled (Ally Sheedy), she finds it emotionally impossible to let him go. Their father (Robert Prosky) is totally against the move too. Lily's situation only gets worse when the happy couple come to visit, causing her to have the best and worst memories of her brother.

"Once and Again" (10 p.m., WXYZ-TV, Channel 7, ABC). Tough love. Patrick Dempsey returns in the role of Lily's (Sela Ward) mentally disturbed brother Aaron. Ally Sheedy guest-stars as the psychiatric patient Aaron has fallen in love with.

Perhaps The Bachelor is the ideal lead-in ABC has been seeking for Once and Again (10 p.m. ET/PT). Let's hope so, because tonight's episode features the return of Patrick Dempsey, one of many actors who should have an Emmy by now for their work on Once.

Love Story - In a moving episode, Patrick Dempsey returns as Lily's sensitive, disturbed brother, Aaron.

Aaron has fallen in love with Mariam (Ally Sheedy), an expressive and volatile mental patient he met in their board-and-care residence. He has been approved for an independent-living program, and he wants Miriam to move in with him, but her father, a retired cop (Robert Prosky), is against the arrangement. So is Lily, especially after Aaron and Miriam descend on the Sammlers for an overnight stay. The unsettling occasion floods Lily with memories of her brother's happy youth and painful teen years.

Then, on the March 25 episode, Lily and Judy's brother Aaron stops by to tell his family that he's in love and is ready to move out of the group home and live independently. That leads Lily to flash back to his very happy childhood, and his very turbulent teen years, when his illness first appeared. I'm not exaggerating when I tell you to grab a whole box of Kleenex for this one!

Aaron tells Lily he wants to move in with a mentally disabled woman (Ally Sheedy), but Lily finds it difficult to let him go.

Ally Sheedy Guest Stars as Aaron's Girlfriend

"Aaron's List of Dreams" - When Lily's brother, Aaron, expresses a desire to move out on his own with a young woman who is also mentally disabled (Ally Sheedy), she finds it emotionally impossible to let him go, on "Once and Again," MONDAY, MARCH 25 (10:00-11:00 p.m., ET), on the ABC Television Network.

"Once and Again" stars Sela Ward as Lily Sammler, Billy Campbell as Rick Sammler, Julia Whelan as Grace Manning, Shane West as Eli Sammler, Meredith Deane as Zoe Manning, Evan Rachel Wood as Jessie Sammler, Susanna Thompson as Karen Sammler, Jeffrey Nordling as Jake Manning, Marin Hinkle as Judy Brooks, Steven Weber as Samuel Blue and Ever Carradine as Tiffany Porter.

Guest starring are Patrick Dempsey as Aaron, Ally Sheedy as Miriam Miller, Winnie Holzman as Shelley, E.P. McKnight as Annie, Adrian Ricard as Annie's mom, Robert Prosky as Herb Miller, Dale E. Turner as the singing man, Esther "Tita" Mercado as Macey, David Stifel as the patient and Great Sesheta as the nurse.

"Aaron's List of Dreams" was written by Richard Kramer and directed by Dan Lerner.

"Once and Again" is broadcast in 720 Progressive (720P), ABC's selected HDTV format, with 5.1-channel surround sound.

This program carries a TV-PG,S parental guideline.

In this moving episode, Patrick Dempsey returns as Lily's sensitive, disturbed brother, Aaron, who has fallen for an expressive and volatile mental patient named Miriam (Ally Sheedy), whom he met in their board-and-care residence. Aaron has been approved for an independent-living program, and he wants Miriam to move in with him. Her father, a retired cop (Robert Prosky), is against the arrangement. So is Lily, especially after Aaron and Miriam descend upon the Sammlers for an overnight stay. The unsettling occasion floods Lily with memories of her brother's happy youth and painful teen years. Shelley: Winnie Holzman.

Cast: Billy Campbell, Sela Ward, Meredith Deane, Julia Whelan, Marin Hinkle, Patrick Dempsey, Ally Sheedy, Robert Prosky, Winnie Holzman

ALLY SHEEDY (!!!) will guest-star on "aaron's list of dreams" (march 25th) as his girlfriend, who may be mentally challenged. the episode finds aaron wanting to move in with her (and out on their own together), and lily struggling to cope with his decision.