INDIAN SUMMER is an impressionistic play where the scenes move in-and-out and overlap with each other.

In Little Falls, Minnesota, a group of friends gathers at a private house in a birch grove along the Mississippi River. They share a weekend enjoying the last days of an Indian Summer.

CECILIA McCORMICK (CC) is a bipolar artist, working on her Hortus Siccus—a collection of dried, pressed flowers which she has collected along the Mississippi. She is waiting for the arrival of her brother, SAM McCORMICK, a successful and controversial artist, and his Life Partner, JIMMY, a feature writer at a mid-west foodie magazine who she has invited for a weekend soiree. CC has stopped taking her bipolar medications and is currently experiencing visitations from an apparition of CHARLES A. LINDBERGH.

Jimmy expresses his discomfort with CC's weekend get-togethers because she doesn't respect his and Sam’s relationship. (Jimmy and CC shared a brief encounter before Jimmy met Sam.) Sam assures Jimmy that the two of them can leave if the weekend gets ugly.

CC invites GLADYS MAUER, a busy-body who runs the gift shop at the Charles A. Lindbergh Museum, where CC sells her pressings and prints to the weekend cook-out because she knows it will make her bother and Jimmy uncomfortable.

Sam and Jimmy arrive in the Birch Grove, soon followed by Dan Foley, CC's current boyfriend. Between the three men, there are introductions and discoveries. CC appears disclosing her apprehensions and her continued attraction to Jimmy. Gladys arrives, and the weekend festivities begin. It is revealed that Sam's dog Golda, which CC was taking care of, drowned in the river. Sam believes that CC was incapable of rescuing Golda and let the drowning happen because of her illness.

Act Two begins with playing the board game "The Game of Life." CC is present but has removed herself from playing the game. Jimmy explains to Gladys how he goes about writing his "Writing in Restaurants" magazine articles. Sam and Dan flirt-up a storm. Sam wants to paint Dan—which means that Dan strips naked, Sam applies paint to his body, and then Dan presses his body onto a piece of canvas; later, Sam will accentuate the pressing.

Eventually, the guys get drunk and go skinny dipping while CC plays her tuba.

The next morning, Sam and Dan prepare for the painting.

CC, at the river’s edge, works on her Hortus Siccus and has another encounter with Lindbergh. CC wants the Aviator to go away and stop ghosting her. He makes her feel crazier than she is. She wants heroines, not heroes.

Sam and Dan have an encounter that doesn't end well. Sam is devoted to Jimmy and unsure of Dan's commitment to his sister.

Jimmy joins CC at the edge of the Mississippi. Their encounter is aggravating to CC, who doesn't understand why Jimmy chose Sam over her. Jimmy is as apologetic as he can be; he also realizes that CC hasn't been taking her medications, and her level of jealously concerning her bother has peaked.

Jimmy slips on the river bank's edge and falls into the waterway. Jimmy can't swim, and CC lets him drown as she calls for Sam’s dog, Golda.