You can see Rhona Mitra in her role as Mercy Graves in the 2nd episode of Supergirl season 4, titled “Fallout”. The synopsis for the episode reads:
“CHAOS IN NATIONAL CITY — A shocking revelation causes chaos in National City. Supergirl (Melissa Benoist) sets out to capture Mercy Graves (guest star Rhona Mitra), while Alex (Chyler Leigh) takes control at the DEO. Back at CatCo, Kara decides to write an investigative story on Mercy and asks Lena (Katie McGrath) if she can interview her as Mercy has ties to the Luthors. Meanwhile, Brainy (Jesse Rath) meets Nia (Nicole Maines) and the two end up in a precarious situation.”
Harry Jierjian directed the episode with story by Dana Horgan and teleplay by Maria Maggenti & Daniel Beaty.