Common Side Effects episode 4 ending explained: Is the assassin able to kill Marshall?

Common Side Effects episode 4 premiered on Adult Swim on February 16, 2025. Titled Dumpsite, the episode puts both Marshall and Frances in quite the big positions where going forward nothing will be the same anymore. Following last week's events, the episode focuses on Marshall trying to grow the blue mushroom once again while also trying to maintain a low profile.

Disclaimer: Spoilers for Common Side Effects episode 4 and some gore content to be mentioned below. Reader's discretion is advised.

Common Side Effects episode 4 features a reunion between Marshall and Frances as it turns out the drug did end up working on Sonia. However, the episode's ending also features an assassination attempt on Marshall's life. While Marshall is able to escape the assassination attempt, it does leave him and Frances is quite the position that will certainly be expanded upon in the upcoming episodes.

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What happened in Common Side Effects episode 4?

Marshall tries to regrow the mushroom

A still from Common Side Effects (Image via Adult Swim)

Following last week's events and being on the run, Marshall travels to the countryside where he tries purchasing a property that's near a dumpsite. There, he meets a police officer and a real estate agent, and while they are suspicious about why he wants to buy a property that's near a dumpsite, he eventually gets it without any questions asked and sets up shop there.

His goal is to make the place have the same sort of climate such as Peru so he can continue to grow the mushrooms. While initially he is able to make some steady progress, the experiment ends up not working as the mushroom's fail to grow in the environment he is in. However, he ends up hearing gunshots near him and decides to investigate.

There he finds that his neighbour is a redneck shooting guns and that he is also the nephew of the police officer that he had met. He warns Marshall to look out for his step as they have laid down mines all over the area to hunt wild boar. Marshall then bids him goodbye and goes away. He then also sends a letter to Sonia asking her to get better.

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The drug ends up working on Sonia

Sonia in Common Side Effects (Image via Warner Bros. Entertainment)

Frances ends up having quite the weird time as well in Common Side Effects episode 4. At the start of the episode, she invites Nick to Switzerland as her boss Rick had some work there. However, after a passionate night, Nick asks Frances to marry her which leaves Frances quite confused about her feelings. At the same time, she receives a call from her mother.

In the last episode, Frances had given her mother the blue mushroom as a cure for her illness. While it seemingly didn't work then, Common Side Effects episode 4 did confirm the fact that Frances's mother had made full recovery and she was feeling okay as well. This leads to Frances dropping everything in Switzerland and going to meet her mother.

When at her mother's place, Frances gets emotional to see that Sonia has made full recovery. Sonia then shows Frances the letter she had received from Marshall and then Frances decides to ultimately go and meet Marshall at where he is posted up right now.

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Copano and Harrington get suspicious of Frances

Copano and Harrington in the show (Image via Adult Swim)

Copano and Harrington get to do a tiny bit more in Common Side Effects episode 4 as well. In the episode, they can be seen investigating the blown up car they had recently found and tracking the rare color that the car had, which directly led them to Rick. However, with Rick being out of the country, they had to go to the service shop he used to take his car at.

There, the mechanic reveals that Rick barely took out his car and that it was his assistant who used to drive it around. After much investigating, Copano and Harrington are then able to figure out Frances's identity and that she is also connected to Marshall as well.

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Frances and Marshall reunite

Frances in Common Side Effects (Image via Adult Swim)

Frances is then able to track down Marshall and she tells him that she trusts him in Common Side Effects episode 4. Marshall then invites her into his house but he notices that someone is spying on him. Marshall then tells Frances that he will be back in a bit and he goes outside to investigate only to be assaulted by an assailant. However, Marshall is able to escape from him.

The assassin then runs after him and steals his neighbor's bike to chase him. The two end up in the woods with the neighbors behind the assailant as well. Frances, who gets suspicious, also goes after Marshall and when the assailant catches up to him, he goes to shoot Marshall but accidentally steps on a landmine which blows him up to smithereens.

Common Side Effects episode 4 then ends with Frances at the spot as well, holding a dismembered leg of the assailant.

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