Sengakuji (泉岳寺), established in 1612, is a Soto Zen temple which is a 2-minute walk from Sengakuji Station of the Toei Asakusa Line or the Keikyu Line, which is one station north from Shinagawa Station.
This temple is famous for its graves of 47 samurai who avenged their lord's death in 1702. This incident was later fictionalized as Chushingura, which has been a very popular subject in the field of novels, plays, movies and the like in Japan. (Ask google if you want to know the plot.)
Chumon was created in 1836. It literally means "the middle gate," and in fact WAS the middle gate, but the first gate is lost now so it's the first gate at present.
Sanmon, the main entrance of the temple, was erected in 1832.
Hondo, or the Main Hall, is post-war reconstruction. The previous Hondo was burnt down in WWII.
Kodo, literally meaning "auditorium", is a half-Japanese-half-Western building completed in 1925.
This is the graveyard of 47 samurai (known as gishi or roshi) who avenged their lord's death in 1702.
The Gishi Festival is held to praise the 47 samurai in early April and on December 14 every year.