Death has hired serial killers to do some of his work for him. in exchange, they receive a monthly dose of (temporary) immortality. if they do not do the work correctly or if they kill someone before their time, then they do not receive the immortality, and are given an impossible task. if they survive said task, then they get another chance.
Holy crap, that's awesome.
These twin siblings found out that they come from a bloodline of secret agents, and that their "father" isn't actually their father. Also, they don't have a father. Miriam is a lesbian and their other mom was sent out of the country on a special mission the day after they were born (sperm donor), so the ladies asked their closest friend, the twins' "dad", to fill in so that Miriam wouldn't have to deal with being an agent and two kids on her own.