Lily and Marshall are both ecstatic to have another couple to hang out with. They invite Robin and Barney for an evening together, which goes awry; Lily and Marshall become clingy and dependent, prompting Barney and Robin to "break up" with them. Eventually, Barney and Robin realize they miss Lily and Marshall, and they all profess their desire to be a foursome. When Barney and Robin's relationship starts to falter, Lily "comes out of retirement" to break them up. She executes an elaborate plan to get them into a big argument; this fails, but they finally realize they don't work as a couple.