Sukadeva Gosvami is narrates the story of Rantideva as follows:

Rantideva is famous in both this world and the next, for he is glorified not only in human society but also in the society of the demigods.Rantideva never endeavored to earn anything. He would enjoy whatever he got by the arrangement of providence, but when guests came he would give them everything. Thus he underwent considerable s...uffering, along with the members of his family. Indeed, he and his family members shivered for want of food and water, yet Rantideva always remained sober. Once, after fasting for forty-eight days, in the morning Rantideva received some water and some foodstuffs made with milk and ghee, but when he and his family were about to eat, a brāhmaṇa guest arrived.

Because Rantideva perceived the presence of the Supreme Godhead everywhere, and in every living entity, he received the guest with faith and respect and gave him a share of the food. The brāhmaṇa guest ate his share and then went away.
Thereafter, having divided the remaining food with his relatives, Rantideva was just about to eat his own share when a śūdra guest arrived. Seeing the śūdra in relationship with the Supreme Personality of Godhead, King Rantideva gave him also a share of the food.When the śūdra went away, another guest arrived, surrounded by dogs, and said, "O King, I and my company of dogs are very hungry. Please give us something to eat."

With great respect, King Rantideva offered the balance of the food to the dogs and the master of the dogs, who had come as guests. The King offered them all respects and obeisances. Thereafter, only the drinking water remained, and there was only enough to satisfy one person, but when the King was just about to drink it, a caṇḍāla appeared and said, "O King, although I am lowborn, kindly give me some drinking water."
Aggrieved at hearing the pitiable words of the poor fatigued caṇḍāla, Mahārāja Rantideva spoke the following nectarean words.

I do not pray to the Supreme Personality of Godhead for the eight perfections of mystic yoga, nor for salvation from repeated birth and death. I want only to stay among all the living entities and suffer all distresses on their behalf, so that they may be freed from suffering.By offering my water to maintain the life of this poor caṇḍāla, who is struggling to live, I have been freed from all hunger, thirst, fatigue, trembling of the body, moroseness, distress, lamentation and illusion.Having spoken thus, King Rantideva, although on the verge of death because of thirst, gave his own portion of water to the caṇḍāla without hesitation, for the King was naturally very kind and sober.Demigods like Lord Brahmā and Lord Śiva, who can satisfy all materially ambitious men by giving them the rewards they desire, then manifested their own identities before King Rantideva, for it was they who had presented themselves as the brāhmaṇa, śūdra, caṇḍāla and so on.

King Rantideva had no ambition to enjoy material benefits from the demigods. He offered them obeisances, but because he was factually attached to Lord Viṣṇu, Vāsudeva, the Supreme Personality of Godhead, he fixed his mind at Lord Viṣṇu's lotus feet.

King Rantideva was a pure devotee, always Kṛṣṇa conscious and free from all material desires, the Lord's illusory energy, māyā, could not exhibit herself before him. On the contrary, for him māyā entirely vanished, exactly like a dream.

All those who followed the principles of King Rantideva were totally favored by his mercy and became pure devotees, attached to the Supreme Personality of Godhead, Nārāyaṇa. Thus they all became the best of yogis.

--SB.9.21.2-18 (summarized)