He agreed to the plan of his own accord.
The underlined idiom means-

• He agreed to the plan of his own accord.
- The underlined phrase means- ক) Voluntarily

• Of one's own accord (idiom)
- English Meaning: Voluntarily or without outside intervention.
- Bangla Meaning: স্বেচ্ছায়।

• Ex. Sentence: He agreed to the plan of his own accord.
- Bangla Meaning: সে স্বেচ্ছায় পরিকল্পনায় রাজি হয়ে গেল।

• অন্যদিকে,
- Unhesitatingly - অকুণ্ঠচিত্তে; দ্বিধাহীনভাবে।
- Reluctantly - অনিচ্ছাপূর্বক।
- Unconsciously - নির্জ্ঞাতভাবে; অবচেতনভাবে।

Source:
1. Oxford Dictionary.
2. Merriam-Webster Dictionary.