when do mark and lexie die in grey's anatomy
In season 8, While flying with fellow surgeons to separate conjoined twins.
Lexie Grey
Mark holds Lexie's hand and professes his love for her, telling her that they will get married, have kids, and live the best life together as they are "meant to be". While fantasizing about the future that she and Mark could have had together, Lexie succumbs to her injuries and dies moments before Meredith arrives. The remaining doctors are left stranded in the woods waiting for rescue, with a devastated Meredith crying profusely and Mark refusing to let go of Lexie's hand.
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Lexie Grey [SEP] Season 8
Days later, Lexie is named as part of a team of surgeons that will be sent to Boise to separate conjoined twins, along with Mark, Meredith, Derek, Cristina and Arizona Robbins (Jessica Capshaw). While flying to their destination, the doctors' plane crashes in the wilderness and Lexie is crushed under debris from the aircraft but manages to alert Mark and Cristina to help her. The pair try in vain to free Lexie, who realizes that she is suffering from a hemothorax and is unlikely to survive. While Cristina tries to find an oxygen tank and water to save Lexie, Mark holds Lexie's hand and professes his love for her, telling her that they will get married, have kids, and live the best life together as they are "meant to be".