From MangaUpdates:

A beast tamer without a familiar, Lock had been supporting his friends in an S Rank party until they finally decided to use him as a decoy and abandoned him. After a conversation with the Fenrir his party was attacking, it chose to serve him as his familiar. By using and expanding the [Mini Monster Garden], Lock gathers more and more familiars to form the strongest- and cutest- party ever. Behold, a fantasy where men and beasts go on adventures together.