Marmaduke, who has been eagerly anticipating Christmas for a good long while now, opens his front door to find not the bearded red coat-wearing man in a sleigh (Santa) he had been hoping for, but a spindly-limbed alien. Marmaduke runs his preconceived checklist of Santa characteristics by his owner-man, who confirms them. It is anyone's guess as to why the alien was knocking on their door, or why Marmaduke opened the door, or HOW Marmaduke opened the door, or how Marmaduke communicated "Are you sure Santa has a red coat, beard and a sleigh?" to his owner-man, but I wouldn't recommend thinking too hard about any of this.