The Superior Vena Cava

After a heated argument (wherein neither can say "I love you", but both mean it), Castle decides that he and Beckett have an expiration date. But a killer's on the loose, one who specializes in ending the suffering of those harboring unrequited love. Soon, Beckett will be left with blood on her hands: she's always held Castle's heart, but soon that may extend beyond the metaphoric.